1. Field of the Invention
The present invention relates to a process for the preparation of a magnetic recording disk, more particularly to a magnetic recording disk provided with a center core in the central portion.
2. Description of Prior Art
A magnetic recording disk (i.e., magnetic recording medium in the form of a disk) of a relatively small size has been recently developed as a video floppy disk.
The magnetic recording disk comprises a nonmagnetic support and a magnetic recording layer provided thereon. In the case of a magnetic recording disk provided with a center core in the central portion, a synthetic resin is generally employed as a material of the nonmagnetic support and the center core. Such synthetic resin has an advantage of a moderate flexibility but has a defect with respect to heat shrinkage which brings about a dimensional change of the resulting disk. In detail, the heat shrinkage of the synthetic resin means an irreversible dimensional change of a magnetic recording medium which is observed when the medium is employed, stored or transferred in an atmosphere of high temperatures.
If the dimensional change of the magnetic recording medium takes place between the recording procedure and the reproducing procedure, the recording medium suffers a phenomenon of so-called "tracking deviation" in the reproduction procedure to cause lowering of reproduction output, and in an extreme case, the reproduction operation cannot be carried out.
For avoiding such heat shrinkage of the synthetic resin, it has been proposed to beforehand subject the support to heat treatment to reduce the heat shrinkage ratio of the support and then prepare a magnetic recording medium using thus heat-treated support or to subject a web of continuous length for the preparation of a magnetic recording medium to a heat treatment in the preparation of the medium (i.e., before or after the procedure of coating a magnetic paint over the support). For the same purpose, it has been also proposed to subject a web roll of continuous length to a heat treatment in its roll state.
However, the above-mentioned proposals still remain various problems. For example, in the case of heating a web of continous length, the heat shrinkage can be reduced in the width direction but can be hardly reduced in the longitudinal direction. Also in the case of heating a web roll of continuous length, the heat shrinkage in the longtudinal direction can be hardly restrained, and additionally a difference of the heat shrinkage is produced between in an outer part of the roll and in an inner part near the center of the roll because of the difference of internal pressure therebetween.
As a measure for improving the above-mentioned methods, Japanese Patent Provisional Publication No. 59(1984)-12733 describes that the web roll is initially cut to give a support in the form of sheet and then plural sheets in the superposed form are heated to reduce the heat shrinkage thereof. This measure can serve to reduce the heat shrinkage in the case of, for example, a floppy disk of 8 inch or 5 inch which is not provided with a center core in its central portion. However, in the case of a floppy disk provided with a center core made of a synthetic resin (e.g., a video floppy disk), the center core having been subjected to no heat treatment still suffers from heat shrinkage. Therefore, this measure does not appear to be satifactory.